Scheiding van Microalgen en Cyanobacteriën met behulp van op Cellulose Nanokristallen Gebaseerde Flocculanten via Sedimentatie en Drijvende Lucht Flotatie (DAF) Scheidingstechnieken

Microalgae are a promising source of biomass, rich in lipids, proteins, and carbohydrates, with applications in nutrition, energy, and pharmaceuticals. However, they also present significant challenges. Eutrophication and climate change can trigger algal blooms that degrade water quality. When these blooms are composed of toxin-producing species, they form harmful algal blooms (HABs), which are increasing worldwide. A major hurdle in microalgae biomass production is harvesting, which can account for up to 30% of total costs. Low biomass concentration (0.5 - 5 g L-1) and small cell size (2 - 5 μm) of microalgae limit centrifugation and filtration separation. Aggregating the cells into flocs can improve sedimentation. Flocculants, such as metal salts, polyacrylamides are often used but can cause biomass contamination. Chitosan is an alternative, but its performance is limited due to coiling in high-ionic strength medium. Cationic cellulose nanocrystals (CNCs) are promising microalgae flocculants due to their high aspect ratio favoring cell attachment and flocculation due to the larger surface area available, conformational rigidity due to crystallinity, and presence of easily modifiable hydroxyl groups. However, when used with marine microalgae, CNCs formed small flocs that can limit sedimentation. Harvesting could be improved by combining CNCs with Dissolved Air Flotation (DAF), a widely used technique for water treatment. DAF uses microbubbles to lift flocs, improving separation. DAF also offers potential for managing HABs, as bloom forming cyanobacteria resist settling in lakes. While conventional flocculants pose toxicity risks, bio-based CNCs with DAF could overcome buoyancy, enable rapid biomass removal, and reduce bloom recurrence. DAF improved marine microalgae harvesting efficiency with pyridinium modified CNCs (PYR-CNCs), achieving up to 20 % higher separation at fivefold lower doses than sedimentation. DAF outperformed sedimentation by efficiently removing small flocs, required less flocculation time and was observed to be insensitive to variations in floc size, when used in combination with different flocculants. Overall, DAF was effective in harvesting microalgae, especially when sedimentation is limited by small flocs. Modified CNCs flocculate microalgae but often remain in the biomass, potentially affecting quality and downstream use. To enable removal from the biomass, pH-responsive zwitterionic CNCs (zCNCs) were synthesized. Unlike PYR-CNCs, the zCNCs were cationic at low pH for flocculation and anionic at high pH for detachment. At high pH, flocs formed with zCNCs redispersed into individual cells, indicating flocculant removal from biomass. Unlike PYR-CNCs, zCNCs demonstrated a detachment capability, enabling potential biomass recovery. PYR-CNCs were successfully used to remove cyanobacteria Microcystis aeruginosa found in freshwater blooms. The study's novelty lies in combining PYR-CNCs with DAF as a sustainable in-lake treatment. DAF removed up to 99% of Microcystis within two minutes of flocculation. For a small recreational lake with Microcystis, the simulation of accumulated residual CNCs post-DAF treatment cycles over a period of time, were observed to be non-toxic to aquatic organisms-Daphnia and C. vulgaris. Overall, these results show that PYR-CNCs offer a safe and sustainable alternative for in-lake water remediation when used with DAF.